Why use this calculator
Intermittent fasting is one of the most researched approaches to weight management and metabolic health. This calculator tells you exactly when to start and stop eating based on your schedule and chosen protocol — taking the guesswork out of your fasting window.
How to use it
Choose your IF protocol (16:8 is the most popular beginner option) and enter your preferred start time for eating. The calculator shows your eating window, fasting window, and break-fast time. Adjust the start time to fit your lifestyle — most people prefer 12pm–8pm.
The formula
Eating Window = 24 − Fasting Hours. Start Eating + Eating Hours = Stop Eating. Fasting Window = Stop Eating to Start Eating next day.
Worked examples
16:8 protocol, start eating at 12pm
Eat 12pm–8pm, fast 8pm–12pm (16 hours)
18:6 protocol, start eating at 1pm
Eat 1pm–7pm, fast 7pm–1pm next day (18 hours)
When people use it
- Starting intermittent fasting for the first time
- Adjusting your eating window to fit work hours
- Planning meals and meal prep
- Combining IF with calorie counting
Tips
- Water, black coffee, and plain tea don't break a fast
- 16:8 is ideal for beginners — most of the fasting occurs during sleep
- Consistency matters more than protocol — the best IF schedule is one you can stick to
- Break your fast with a protein-rich meal to preserve muscle mass
- Fasting may not be suitable if you have diabetes, are pregnant, or have a history of eating disorders
Questions people ask
- Does coffee break a fast?
- Black coffee (no milk, cream, or sugar) doesn't meaningfully break a fast and may actually enhance fat burning. Bulletproof coffee (with butter or MCT oil) does add calories and breaks the fast.
- What can I drink during a fast?
- Water, sparkling water, black coffee, and plain herbal tea are all safe during a fast. Avoid anything with calories, including diet sodas with artificial sweeteners if your goal is metabolic health.
- Is intermittent fasting safe?
- For most healthy adults, intermittent fasting is safe. It's not recommended for pregnant or breastfeeding women, people with type 1 diabetes, those with a history of eating disorders, or people who are underweight. Consult your doctor if unsure.